As integration density increases, logic circuits and memory chips are becoming ever smaller and contain an increasing number of electrical terminals in a confined space. Usually, the semiconductor element is placed on a substrate and the latter bears terminal areas which are connected to contact areas of the semiconductor element by means of bonding wires. During the contacting of such pieces of bonding wire with terminal areas and contact areas, high precision is required, since there is little space available. In this case, wires with a diameter of between 17 and 100 μm. are used for thin wire bonding, wires with a diameter of between 100 and 500 μm. are used for thick wire bonding. The corresponding terminal areas are metallized areas, for example of gold, copper or aluminum. The bonding wires may likewise consist of one of these materials and are brought to the desired cross section by repeatedly drawing them through diamond dies.
Thermocompression, ultrasonic and thermosonic methods are used in most cases for the automatic bonding. The three methods mentioned are described, for example, in a manuscript from the technical university of Cottbus, which is available on the Internet at “www.tu-cottbus.de/MST/lehre/scripte/UES-Bonden.pdf” with the title “Elektrische Kontaktierungen in Mikrosystemen—Drahtbonden” [Electrical contacting in Microsystems—wire bonding].
In the case of the thermocompression method, also known as ballhead bonding or nailhead bonding, the joining of the bonding wire to the terminal area takes place by exposure to heat together with a compressive force. The connection created is radially symmetrical.
In the case of ultrasonic bonding, firstly a bonding wire is pressed onto a terminal area by a tool (wedge) and then ultrasound is introduced into the bonding tool, so that the bonding wire moves over the terminal area in a rubbing manner with a frequency of approximately 40 kHz to 150 kHz.
In a first phase of the bonding process, any kind of disruptive materials are thereby rubbed away by the friction between the parts being joined. In the second phase, in which the materials to be connected lie right against one another, the temperature increases as a result of friction until the areas being joined lie against one another virtually without any distance between them and the increased temperature causes diffusion effects to occur. In this phase, the parts being joined adhere to one another and the joining tool comes away from the bonding wire and brushes over its surface, which leads to further heat input, which anneals the connection and prevents the occurrence of brittle locations.
Modern bonding devices provide as a measured variable during the bonding process on the one hand the reflected ultrasound output, on the other hand the deformation of the bonding wire which is lowered onto the terminal area. Correspondingly measured parameters show a typical reproducible variation over the bonding time.
Modern bonding devices operate in an automated manner and are programmable, so that the bonding of a semiconductor element can take place at bonding rates of approximately 10 terminals per second. In the case of this process, defects repeatedly occur, manifested by modules which do not operate or do not operate sufficiently reliably.
Various methods of attempting to lower failure rates for the automated bonding process are known.
For example, the so-called Motionblitz system is known, in which the tool is photographed with a high-speed camera during the bonding operation, in order to allow wrong movements of the tool, but also of the restraint of the workpiece, to be detected by a subsequent analysis and eliminated by optimization of the process.
Furthermore, the website “www.Semikonduktorfoptech.com” discloses a “Wirebonding process control” system from the company F & K Delwotech GmbH, in which the bonding wire deformation and the reflected ultrasound output of the ultrasonic bond are continuously measured during the bonding process. It can be detected from the variation of the deformation and the ultrasound output whether the bonding operation is progressing typically with a good result, or whether the case concerned is a special case, for example as a result of contamination of the substrate surface or the like. The measured values are assigned to the respectively measured semiconductor element or module and stored in a memory device, so that during the subsequent test a failed part can still be retrospectively assigned the measured values of the bonding operation. It can then be found out whether the failure was caused by an irregularity in the bonding process and possible readjustment of the process is required.
Immediate discontinuation of the bonding process is also given as a possibility, if it is found by analysis of the measured data during the bonding process itself that there is a fault.
The method described allows the bonding process to be tracked within narrow limits on the basis of the measured variables required. However, only faults which are specifically associated with the mechanical operation of ultrasonic bonding, and defects thereby occurring, are detected.
However, it would be desirable to allow a comprehensive defect analysis which nevertheless allows measured variables to be assigned in each case to the individual semiconductor element both during the bonding operation and retrospectively.